The present invention relates in general to imaging acquisition devices and in particular to high detective quantum efficiency X-ray detectors.
Imaging systems such as X-ray imaging are widely used in various fields of life and industry. For example, X-ray imaging is commonly used in non-invasive inspection of objects such as cargo containers, luggage, bags, briefcases and the like, to identify hidden contraband at customs and ports. The contraband may include hidden guns, knives, explosive devices and illegal drugs or goods.
Detectors used in existing cargo scanning systems employ scintillator crystals to detect X-rays. Atoms within the scintillator interact with incident X-ray photons and are raised in energy. When the energetically excited atoms in the scintillator decay back to their ground state they emit light. The light is detected by a photodiode placed behind the scintillator. The photodiode generates an electrical signal proportional to the flux of X-rays absorbed in the scintillator.
The detector structure typically consists of a linear array of several hundred individual detector elements each containing a block of scintillator material and a photodiode. The individual detector elements form a strip of pixels, each pixel being of the order of, for example, 5 mm×5 mm in size. The detector strip is illuminated by a fan beam of high energy X-rays (typically >1 MeV photons), from an accelerator or an isotope source.
An important requirement in an X-ray detector is that it should be able to efficiently absorb the incident X-ray photons. If an X-ray photon passes through the detector without interacting, then no information is obtained from that photon. It has simply passed through the detector after irradiating the test object.
A fundamental measure of the performance of a detector is its detective quantum efficiency (DQE). This is a measure of the fidelity of the detector in capturing and transferring image information. The range of DQE is 0<DQE<1, where the value of 1 implies that all the image information in the incoming X-rays is captured and no noise is added. Even under theoretically perfect conditions, DQE cannot exceed the fraction of the X-ray photons that are absorbed in the detector. Thus, if only 30% of the photons are absorbed, DQE cannot exceed 0.3.
The scintillator materials that are used most frequently in current high energy X-ray detectors are crystalline ceramics such as cadmium tungstate (CdWO4) and bismuth germanate (Bi4Ge3O12) (abbreviated to BGO). These materials combine the properties of high light output when irradiated with X-rays, rapid decay of light output when the X-ray irradiation is stopped, high average atomic number, and high physical density. The latter two parameters are important for the scintillators to be able to effectively absorb high energy X-ray photons which readily penetrate most materials.
Scintillator materials for scintillator-based X-ray detectors are expensive. Cadmium tungstate and BGO are noteworthy among scintillator materials both for the high average atomic number of their constituents, and their high densities (7.9 and 7.13 gm/cc respectively). These properties make them relatively efficient absorbers of X-rays. However, MeV X-ray photons are used for X-ray scanning of cargo container precisely because of their great penetrating power. This means that even with these high density materials, a considerable thickness of scintillator is needed to absorb a sufficient fraction of the X-ray photons entering the detector.
A typical linear detector array used in a cargo container scanning system is made of many individual blocks of cadmium tungstate approximately 5 mm×5 mm×30 mm in size. The blocks are placed with their long dimension aligned with the path of the incident X-rays. The incident X-ray photons therefore traverse a 30 mm thickness of the scintillator. This is sufficient to absorb approximately 40% of the incident high energy X-rays, which limits the maximum possible DQE to 0.4
Detector-grade cadmium tungstate and BGO scintillator crystals cost of the order of $10,000 per kilogram. If a 30 mm thickness of scintillator material is used, around 8 to 10 kilograms of scintillator will be required for a cargo scanning detector, at a cost of $80K-$100K. This is a crippling cost penalty that forces an engineering tradeoff in terms of the amount of scintillator that can be used versus the detector performance. It is a significant component of the total system cost and it has a negative impact upon system sales.
The above cost figure is for the quantity of scintillator materials required to build a single row detector. If a multi-row detector could be used, then much faster scanning speeds would be possible. This would enable faster throughput of containers which is a key to the acceptance of cargo scanning systems in ports worldwide. However, a multi-row detector is currently not economically viable because of the high cost of the scintillator materials.